It's not usually a preference against smiling. First off, it's an expression, usually said to be intimidating, so that in itself is a reason to say it. For another, the dom may want to feel that his/her sub is scared/nonconsenting/etc for the scene to flow as desired. It could also be that it's seen as a look of disrespect- smiling isn't very serious, and dom/mes are 'superiors,' like in the workplace, military, etc.

Agree with Bunny. I allow my pet to smile if she wishes. we will also have times when our play will kinda pause because we are both giggling. BDSM is all about what you WANT it to be. Not what the BDSM social norm is.